1. Field of the Invention
The present invention relates to producing a low-calorie sweetening substance, more particularly, to crystallization of the .alpha.-L-aspartyl-L-phenylalanine methyl ester.
2. Description of the Related Technology
.alpha.-L-aspartyl-L-phenylalanine methyl ester, hereinafter referred to as aspartame, is a low-calorie sugar substitute and is industrially synthesized by several different processes. The synthesized aspartame is isolated most commonly by crystallization, which is followed by dewatering and drying processes. Further treatments or processes can be applied to the aspartame to provide various consumer products.
Crystallization occurs by cooling a solution or by evaporating the solvent thereof. In cooling crystallization, as the solution is cooled, the solute is supersaturated, which causes formation and growth of crystals. Generally, obtaining coarse and uniform crystals is the goal. In theory, this can be accomplished by cooling the solution with an infinitesimal amount of heat transfer.
In U.S. Pat. No. 5,543,554, a method of cooling crystallization is disclosed. An aspartame solution is conductively cooled within a crystallizer without any forced agitation or stirring. The obtained crystals of the aspartame are coarse and uniform, which is desirable. However, since no forced flow is utilized in the cooling crystallization of the aspartame, the crystals obtained within the crystallizer are not easily discharged and it takes a relatively long period of time.
In European Patent Application Laid-Open 523,813 A, an aspartame solution is directly contacting with ice to cool and crystallize the aspartame. The process can reduce the time required in the cooling crystallization. However, as the ice is melting, the aspartame solution is diluted and the final yield of the aspartame crystals decreases.
In U.S. Pat. No. 5,502,238, an alternative method of crystallizing an aspartame is disclosed. This method uses the solubility changes of the aspartame, depending on pH of the solution. However, since acidic solution of the aspartame is neutralized at a relatively high temperature, the aspartame may break down and produce a byproduct such as benzyl-3,6-dioxo-2-piperazine acetic acid (DKP).
There is thus a need for an alternative or improved method of crystallizing aspartame.